At the present time glaucoma is treated in the most serious cases by a surgical operation known as a trabeculectomy. This operation consists, following conjunctival disinsertion, of making an opening in the sclera by cutting out a scleral flap (division in the planes of the sclera as far as the trabeculum) and incising the trabeculum to enable the aqueous humor contained in the anterior chamber to flow out, thus reducing the intra-ocular pressure and limiting the pathological consequences of glaucoma. Such an operation, which has been performed for about fifteen years, generally produces a temporary improvement, but in time, healing of the scleral flap is liable to obstruct the flow of aqueous humor and a renewed increase in the intra-ocular pressure is then noted.
An attempt has been made to overcome this defect by applying antimitotic substances on the scleral flap so as to retard healing. However, the results are variable and it is even possible to observe an excessive flow of aqueous humor which can bring about too great a fall in the intra-ocular pressure.
Another type of operation is currently practiced for treating glaucoma. It consists of putting a valve in place associated with a tube which emerges in the anterior chamber to enable aqueous humor to flow out in the case of an increase in pressure in this chamber. This technique has disadvantages however. In the first place, it requires a complex operation affecting the anterior chamber, with the risk of a deleterious effect on the corneal endothelium, and surgeons who are used to trabeculotemies are reluctant to perform this type of complex and risky surgical operation. Moreover, the tube tends to block up so that the efficiency of the device, which is good after it has been put in place, decreases rapidly with time. In addition, the pipe is sometimes expelled from the anterior chamber. It should be noted that these valves have a relatively complex structure which makes them costly to manufacture.
In addition, several publications have proposed novel techniques which are at the experimental stage (or have remained at the experimental stage on account of basic defects) and which consist of inserting an implant under the scleral flap with a view to preventing the flow of aqueous humor through the incision in the trabeculum from being obstructed too rapidly.
A first technique is described in the following publication "Hyun Bong Bae et al., A Membranous Drainage Implant in Glaucoma Filtering Surgery: Animal Trial, Kor. J. Ophthalmol., vol. 2, 1988, 49-56". It consists of placing a hydrophobic membrane under the scleral flap which penetrates into the anterior chamber by means of an incision in the trabeculum. By virtue of its hydrophobic nature, this membrane opposes the formation of fibroses close to its surface and enables aqueous humor to flow out along it. However the permanent flow thus produced is small and the presence of such a membrane appears to be insufficient to restore the intra-ocular pressure to a normal value. Moreover, its insertion involves an operation on the anterior chamber with risks to the corneal endothelium, this operation being much more delicate than a simple trabeculectomy as already indicated.
The following publication: "M. Kamoun et al., Microtrabeculoprothese, Ophtalmologie, 1988, vol. 2, 227-229", describes a trabeculoprosthesis made of a very hydrophilic porous hydrogel which is put in place in the dry state in the sclera so as to penetrate the incision in the trabeculum by a tapered part. Hydration causes the hydrogel to swell and the incision to be hermetically sealed. The aqueous humor then flows out through pores in the material from the anterior chamber to the sclera. This filtration though a porous material of this type brings about a reduction in the intra-ocular pressure. However, the aforementioned defect, which lies in a rapid obstruction of the flow, is not overcome by this prosthesis, since fibroses which form in the sclera rapidly lead to obstruction of the flow. Moreover, as in the previous technique, insertion of this prosthesis affects the endothelium because a tapered part of the prosthesis is made to penetrate inside the anterior chamber.
U.S. Pat. No. 4,521,210 describes an implant with an elongated shape which is placed above the choroid at the interface between this and the sclera. This implant has a tapered part which penetrates the anterior chamber. It has the same disadvantages as the previous one, even though its long length and the hole and passages provided in it ensure better discharge of the aqueous humor. In addition, its insertion would appear to be extremely delicate, considering the dimensions of this implant and its position at the choroid/sclera interface.
Another implant is described in the following publication "Nancy Michel, A Wick that promotes fluid drainage treats glaucoma successfully in Russia, Ocular Surgery News 1993, vol. 11 (23) p 26". This implant consists of a bundle of reabsorbable fibres which is put in place under the scleral flap facing the incision in the trabeculum. The inventors have very little information on this technique which has been proposed very recently. However, it is probable that such an implant of a biological origin (pig), buried in the sclera, will experience the same blockage problems as the previous implants. Moreover, displacements in the opening, or even expulsion, are to be feared.
Another implant is described in International Application PCT/US 90/06216. This implant, inserted in the sclera, does not resolve the aforementioned defect of blockage of the flow by the fibroses which form in this tissue.
It should be noted that all the known implants previously referred to are put in place following a trabeculectomy during which the trabeculum is incised.